Heretofore, a lead storage battery has been generally employed as a secondary battery. Recently, however, a lithium-ion battery which has extremely high power density as an electricity storage medium and can be downsized has been extensively employed. A lithium-ion battery, e.g., has a 4V charging voltage per cell, which is nearly twice as large as that of the lead storage battery. In other words, a lithium-ion battery has the advantage that half as many series-connected lithium-ion batteries as series-connected lead storage batteries are capable of producing the same charged voltage. Further, in order to obtain identical capacity and an equivalent charged voltage, a lithium-ion battery permits a downsized and weight-saved secondary battery to be built as compared to a lead storage battery.
FIG. 8 shows a structure of a laminate-type lithium-ion battery that is one representative type of lithium-ion batteries. A lithium-ion battery 1 has a structure where an insulating separator 4 is inserted between a positive-terminal material 2 such as, lithium cobaltate (LiCoO2), lithium manganate (LiMnO2) or the like, and a negative-terminal material 3 such as graphite (carbon) or the like, and then some of terminal material units thus assembled are stacked into a laminate body 5, which is, then sealed together with an electrolyte, with aluminum laminates 6 from upper and lower sides thereof. The positive-terminal material 2 and the negative-terminal material 3 are formed with a positive-terminal 2a and a negative-terminal 3a, respectively, and both the terminals protrude outside from where the aluminum laminates 6 are bonded. In the meantime, the lithium-ion battery 1 is often used in the form of a battery stack where a plurality of single cell stacks thus assembled are connected with one another, and there are no particular limitations to the way the terminals are protruded, the shape and material of the terminals, the whole size of the laminate battery.
Despite the extremely high power density as an electricity storage medium, the lithium-cell 1 of the foregoing structure is sensitive to handling conditions, in comparison with the lead storage battery, Ni—Cd and Ni-MH batteries or the like. Particularly, when it is charged or discharged under high temperature, its life is shortened, and susceptible to overvoltage and therefore application of an excessive overvoltage to the lithium cell in charging involves risks of smoking and ignition.
Hereinafter is described a mechanism by which the lithium-cell 1 leads to the smoking and the ignition in the application of the excessive charging voltage with reference to FIG. 9 illustrating a state of the lithium-cell 1 in relation to temperature.
First, when overvoltage is applied, degradation of the electrolyte inside a battery accelerates and then heat is generated and temperature in the battery starts to rise. At the same time, evaporating gases (diethyl carbonate and ethylene carbonate gases) of the electrolyte are generated inside the battery to cause expansion of the aluminum laminates 6. On this occasion, some quantities of the evaporating gases are discharged out of an explosion-proof valve. A separator 4 of the present battery has a double structure and its material is formed from PE (polyethylene) and PP (polypropylene). When an internal temperature of the battery has risen to reach about 120 deg C., the separator 4 thereinside begins to shrink. When the temperature has risen further, a PE separator making up one part of the separator 4 begins to be dissolved at about 135 deg C. Then, a PP separator making up the other part of the separator 4 begins to be dissolved at 165 deg C. At this dissolution of the PP separator, an internal dielectric breakdown of the lithium cell 1 progresses. Further, seals of the aluminum laminates 6, 6 are broken to thereby start a discharge of the internal gases. Thence, the temperature rise progresses rapidly to cause a thermal decomposition of the electrolyte at 250 deg C., and then gases such as CH4, C2H4, C2H6 are generated to break the insulation performance of the separator 4. In due course of time, when internal short-circuiting has begun, a sparking phenomenon acts to satisfy an ignition point, thus ending up the ignition.
As described above, the lithium-ion battery 1 stands up poorly to an overvoltage due to the same utilizing lithium ions and therefore it has been recognized that the application of the overvoltage will lead to the smoking and the ignition if the worst comes to the worst. It has been, however, left unexplained how the input energy (charging power) including impressed voltages and currents is exactly related to the smoking and ignition of the laminate-type lithium-ion battery, and therefore a situation where no fundamental measure is available against the hazard has continued. As an interim measure to solve the safety hazard, a protection circuit for preventing the smoking and ignition of the battery (e.g., refer to Japanese unexamined patent application publication No. 8-222278 and Japanese patent No. 2861879) is normally installed inside a battery charger and a battery pack. In Japanese unexamined patent application publication No. 11-222278, the smoking and ignition of the battery are prevented by prohibiting charging to a secondary battery when a flammable gas or the like is detected.
Further, Japanese patent No. 2995142 discloses a battery charger in which when each battery cell is monitored with respect to its charged voltage and then any battery cells have reached an upper limit of the charged voltage, the battery cells having reached the upper limit are bypassed to continue to pass charging currents to the other batteries to allow all battery cells to be fully charged.
FIG. 10 is a block diagram representing an outline configuration of the lithium cell 1 that is mounted on an electronics device or the like. In the drawing, the lithium cell 1 is configured as a battery pack 10 having a protection circuit 11 incorporated therein. The protection circuit 11 is equipped with, e.g., a current fuse, a temperature fuse and an overvoltage protector to thereby shut off the charging power supplied to the lithium cell 1, thus protecting the lithium cell 1 at the time of overcurrent, overvoltage and abnormal temperature. A battery charger 12 for inputting a charging power to the lithium cell 1 to charge the same is connected with a previous stage to the battery pack 10. The battery charger 12 comprises a stabilized power supply 13 for producing a stabilized electric power and a charging circuit 14 for supplying the charging power to the battery pack 10 using the stabilized electric power 14, thus charging the lithium cell 1.
FIG. 11 is a block diagram representing an outline configuration when the protection circuit is mounted on the lithium battery 1. In the figure, the battery charger 12 for inputting the charging power to the lithium-ion battery 1 to charge the same is connected with the lithium-ion battery 1. The battery charger 12 comprises a constant-voltage and constant-current circuit for stabilizing a charging voltage or a charging current to charge the lithium-ion battery 1 linearly, a pulse charging circuit for supplying a pulse-shaped charging current to pulse-charge the lithium-ion battery 1, or the like to allow these circuits to be arbitrarily selected according to battery performance and its life span.
Specifically, the protection circuit, as shown in FIG. 12, may comprise a comparator 15, an OR circuit 16 and a temperature sensor 17 and monitors a voltage of the charging power output to the lithium-ion battery 1 and a temperature rise of the lithium-ion battery 1 to shut down the battery charger 12 in overvoltage and excessive charging, thus protecting the lithium-ion battery 1. A noninverting control input terminal of the comparator 15 is connected with a connecting line with the battery charger 12 and the lithium-ion battery 1 for the sake of inputting a monitoring target voltage of the charging power. At the same time, a given reference voltage Vref output from a reference electric power source 18 is input to an inverting control input terminal of the comparator 15. When having detected an overvoltage exceeding the reference voltage Vref at the time of an abnormal output or the like of the battery charger 12, the comparator 15 outputs an overvoltage signal S1 from its output terminal. Further, a temperature sensor 17 attached to the lithium-ion battery 1 detects malfunction in excessive charging from the temperature rise of the lithium-ion battery 1 to output an excessive charging signal S2. Further, the overvoltage signal S1 and the excessive charging signal S2 are input to the OR circuit 16 and either the overvoltage signal S1 or the excessive charging signal S2 is output to thereby output a malfunction signal S3 from the OR circuit 16 to the battery charger 12. As a result, when having received the malfunction signal S3, the battery charger 12 shuts off the charging power supplied to the lithium-ion battery 1.
When the protection circuit 11, shown in FIG. 10, fails to operate properly from any cause, however, risks of smoking and ignition cannot be avoided. Hence, the wide use of the lithium-ion battery in fields where high reliability is required, such as in the fields of an electric power source and uninterruptible power supply unit, has lagged behind.
Under the existing situation, it has still been left unexplained how far the process in the above-mentioned smoking and ignition mechanism progresses in response to input energy (voltage and current stresses) such as overvoltage, excessive charging or the like. As a result, both battery makers and device makers using batteries cannot help relying upon the protection circuit against the smoking and ignition. In reality, the protection circuit for monitoring and controlling the battery, however, becomes inoperative in protection performance, if its monitors (a voltage monitor, a current monitor and a temperature monitor) are out of order, or its controller (its control circuit) for receiving signals from the monitors to control the signals received is out of order, or further protection elements (a shut-off switch, a fuse, semiconductor switches such as FFTs, bipolar transistors or the like) for receiving signals from the controller to operate fails to operate properly as a protection circuit. That is, if the worst comes to the worst, there is a possibility of inducing the smoking and the ignition.
Further, the conventional protection device for a secondary battery shown in FIG. 11 does not detect the malfunction until abnormality such as excessive charging or the like has occurred and hence a protecting operation for the battery charger 12 has been often too late. Accordingly, in the event that the temperature rise of the lithium-ion battery 1 progresses rapidly, there has been a possibility to cause the smoking and the ignition.
Additionally, a temperature sensor 17 needs to be attached to the lithium-ion battery 1, having raised a problem that the excessive charging cannot be detected from information only from the battery charger 10 side. Particularly, in the lithium-ion battery 1 mounted on an inside of a portable device, connection terminals for temperature information must be provided other than connection terminals for charging, in order to connect the temperature sensor 17 attached to the lithium-ion battery 1 electrically with the battery charger 10.
Incidentally, in the case of excessive discharging, a secondary battery such as the lithium-ion battery 1 is liable to generate abnormal heat to be broken due to internal short-circuiting if the worst comes to the worst as well as remarkable shortening in life span. Hence, the lithium-ion battery 1 needs to be protected in discharging in the same way as in charging. In Japanese patent No. 2861879, e.g., a secondary battery pack equipped with both excessive discharging protection circuit and excessive charging protection circuit is disclosed.
Further, in the conventional battery charger containing the battery charger disclosed in Japanese patent No. 2995142 and the conventional electric power unit including this sort of the battery charger, charging and discharging control has been practiced based on a battery cell terminal voltage that is information from a detection means or the like or on an output voltage or output current of a charging circuit. Besides, such control has been performed that when the charging circuit is out of order, a protection circuit is allowed to operate. Accordingly, it has been seen as the problem that when functions of the detection means and protection circuit has gone down, the charging and discharging control for the lithium-ion battery fails to operate properly, resulting in lowered performance, life span or the like of the lithium-ion battery. Moreover, it has been seen as the problem, too, that in the worst-case scenario such as a series of chance failures (so-called “if the worst comes to the worst”) occurs, there has been a risk of leading to the smoking and the ignition.
Besides, the lithium-ion battery 1 is charged by applying typically 4.1 V or 4.2V/cell thereto. Accordingly, when charging a lithium-ion battery comprising a plurality of series-connected battery cells, a voltage of a charging voltage/cell multiplied by the number of series-connection is applied to the whole of the lithium-ion battery.
On one hand, at the time of controlling the charging of the lithium-ion battery 1 without interruption under high temperature, a separator for separating positive-and negative-terminals is liable to expand to be holed therein, thus allowing lithium attached to the positive-terminal and metallized to pass through the separator to reach a positive-terminal side, leading to the likelihood to short-circuit the positive and negative-terminals. In the worst case scenario, all other cells can be short-circuited with only one cell remaining non-short-circuited among multiple-cells-series-connected lithium-ion batteries. In the possible worst case scenario like this, the problem has also arisen that a voltage for charging all the battery cells is applied to the one cell only to lead to a high likelihood of causing smoking and ignition.
On the other hand, for a maker side, reflecting the worst case leading to the smoking and ignition, it is general to strengthen a protection circuit function so as to practice a fail-safe design for securing safety even if a function of the protection circuit has been lost. It could not be denied, however, that there are such high risks involved that if a series of chance failures have occurred, due to charging action being performed with a fail-safe function remaining inoperative, the lithium-ion battery rises in temperature to be led to the smoking and the ignition.
As an alternative measure, it might be possible to aim at a perfect safety measure against the worst scenario, which, however, inevitably leads to a sharp cost increase in equipment, making such measure poor in feasibility.